1. Field of Invention
The present invention relates generally to stencil cleaning methods and apparatus, and more particularly to a stencil cleaning apparatus that consistently and evenly applies a fluid, such as solvent, on a web material prior to the web material being placed in a position to clean the stencil.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a circuit board having a pattern of pads or some other conductive surface onto which solder paste will be deposited. The circuit board is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. Once a circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures formed in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard cartridge.
In some prior art stencil printers, any excess solder paste remaining under the squeegee after it has fully traversed the stencil remains on the stencil when the squeegee is returned to its initial position for printing on a second circuit board. Usually, as the squeegee passes the solder paste over the stencil, minute amounts of solder paste seep through the apertures to accumulate at the bottom side of the stencil. This presents various problems, such as the solder paste being inadvertently disposed on the unintended areas of the circuit boards. Also, as the solder paste hardens, it complicates the alignment procedure of a circuit board with the stencil. Therefore, it is highly desirable to remove the excess solder paste that forms on the bottom of the stencil.
U.S. Pat. No. 5,918,544 to Doyle represents one prior art stencil printer having a well-known method and apparatus for cleaning the bottom of the stencil. Doyle discloses a wiping system that is positioned near the vicinity of the stencil and moves beneath the stencil from one end of the stencil to the other end. As the stencil wiper system moves beneath the stencil, it wipes off excess solder paste at the bottom of the stencil.
Specifically, the stencil wiper system includes a paper supply roller containing a roll of paper, a take-up roller, a pair of paper guide rollers, a hollow solvent tube with numerous small openings formed along the length of the tube, and a vacuum plenum for removing excess moisture and hardened solder paste from the paper as it travels underneath the stencil. During a cleaning operation, a paper winder motor rotates the take-up roller to draw paper from the paper supply roller, which passes paper through the pair of paper guide rollers. The hollow solvent tube is located between the paper guide rollers and is filled with solvent by a solvent pump, which causes the solvent tube to squirt solvent through its numerous holes onto the paper as it passes the solvent tube. The solvent impregnated paper is passed to the vacuum plenum, which holds the paper in place as the stencil moves over the paper, thereby cleaning the stencil.
A disadvantage to the system described in Doyle is that solvent may be applied to the paper in an inconsistent and uneven fashion. Specifically, since the pressure of the solvent closer to the solvent source, which is typically introduced at one end of the solvent tube, is far greater than the pressure at the other (opposite) end of the solvent tube, the solvent has a tendency to more completely impregnate the paper closer to the solvent source and barely wet the paper at the other end of the solvent tube. The result is that the more fully solvent impregnated portion of the paper cleans the stencil more effectively than the portion of the paper having less solvent. Another result is that too much solvent may be delivered to the portion of paper close to the solvent source thereby resulting in an excessive amount of solvent being applied to the stencil.